This invention relates to a process for producing sebacic acid from adipic acid. More particularly, this invention relates to a process for producing sebacic acid by carrying out electrolytic condensation of monomethyl adipate.
Electrochemical condensation reactions of carboxylic acids are generally called as the Kolbe reaction and described in such references as H. Kolbe; Ann. 69, 257 (1849), A. C. Brown; Ann. 261, 107 (1891), etc. One example of the Kolbe reaction is the production of dimethyl sebacate from monomethyl adipate, in which the reaction at an anode can be represented by the following equation: ##EQU1##
In order to produce sebacic acid from adipic acid industrially, it is necessary to pass through the following three steps, that is, (1) the first step comprises the production of monomethyl adipate, (2) the second step comprises the production of dimethyl sebacate from monomethyl adipate by electrolytic condensation and separation and purification of dimethyl sebacate from the electrolytic solution containing dimethyl sebacate, and (3) the third step comprises the production of sebacic acid. According to conventional processes, each step can be carried out, for example, as follows. Monomethyl adipate is produced by half esterifying adipic acid with methanol in the absence of a catalyst or in the presence of an acid catalyst (Japanese Patent Appln. Kokai (Laid-open) No. 100024/74). Electrolytic condensation of monomethyl adipate is carried out by dissolving monomethyl adipate in a methanol solution and electrolyzing the solution partly neutralized with an alkali such as potassium hydroxide to yield an electrolytic solution such as a methanol solution containing monomethyl adipate and its alkali metal salt, dimethyl sebacate, and trace amounts of by-products such as dimethyl adipate, methyl n-valerate, methyl .omega.-hydroxyvalerate, methyl allylacetate and the like, and subsequently separating dimethyl sebacate which is an electrolyzed product in the electrolytic solution from the starting monomethyl adipate and its alkali metal salt by extraction procedure (U.S. Pat. No. 3,896,011). Sebacic acid is obtained by hydrolyzing dimethyl sebacate thus separated and purified in the presence of an acid catalyst (U.S. Pat. No. 3,896,011).
In industrial scale production of sebacic acid according to a conventional process, there was the most important defect in the second step of separating dimethyl sebacate from the electrolytic solution containing dimethyl sebacate. That is, as a method for separating dimethyl sebacate from the electrolytic solution containing dimethyl sebacate, there are such methods as distillation, crystallization, extraction and the like. But as to a distillation method, since there is a defect in that dimethyl sebacate and monomethyl adipate produce an azeotropic mixture, it is not preferable to use the distillation method for the separation. Further in a crystallization method, since it is necessary to carry out the procedure at a low temperature, there is a defect in that contamination with monomethyl adipate and its alkali metal salt is inevitable during the crystallization of dimethyl sebacate. Therefore, there has been employed an extractive separation method by adding water, hexane, heptane and the like to the electrolytic solution. But according to said method, if water is used as an extracting agent, there are defects in that a large quantity of water should be used, and the separation of the oil layer and the water layer after the extraction is very difficult due to a very small difference of specific gravities of the two layers; and if an organic solvent such as n-heptane is used as an extracting agent, there is a defect in that it is inevitable to accompany more than a certain amount of monomethyl adipate to the n-heptane layer. According to the most recent method described in U.S. Pat. No. 3,896,011, the separation of highly pure dimethyl sebacate is successfully carried out by using a comparatively small amount of an extracting agent, i.e. by adding an organic solvent, which dissolves dimethyl sebacate but does not dissolve water, together with water to the electrolytic solution and separating after a prescribed contact time. But even in the most recent process as mentioned above, the defects of the conventional processes are not solved fundamentally. That is, the new insertion of water and an organic solvent into the system is inevitable, and the use of more than a certain amount of an extracting solvent is necessary in order to separate highly pure dimethyl sebacate: these are defects of said process.
The present inventors have studied earnestly to overcome the defects of the conventional processes and found that monomethyl adipate in the electrolytic solution can be separated by adsorption with anion exchange treatment, an alkali metal salt of monomethyl adipate can be separated by extraction with an extremely small amount of water or by adsorption with cation exchange treatment as alkali metal ions, and further monomethyl adipate adsorbed on the anion exchanger can easily be regenerated by methanol or water present in the system and alkali metal ions adsorbed on the cation exchanger can easily be regenerated by monomethyl adipate and/or a methanol solution containing monomethyl adipate present in the system. Consequently it has become possible to easily separate dimethyl sebacate from monomethyl adipate and its alkali metal salt and to afford highly pure dimethyl sebacate without introducing newly an organic solvent and water into the system or in some cases by only introducing an extremely small amount of water into the system.